The present invention relates generally to shielding systems for electrical components, and more specifically, to a radio frequency shielding device for a printed circuit board.
Radio frequency shielding in electronic equipment such as mobile telephones, is required to reduce receiver radio frequency interference from digital circuit switching noise and to prevent malfunction of the digital circuits due to locally transmitted radio frequency signals. Conventional shielding devices include a frame soldered on a printed circuit board and a cover attached to the frame. The frame is typically placed onto the printed circuit board along with the electronic components using standard SMD (Surface Mounting Devices) pick-and-place technology equipment on the production line before the printed circuit board goes through re-flow soldering. Optical inspection is then performed to ensure proper placement of the components. After the optical inspection is passed, a functional electrical test is performed on the printed circuit board. If the functional electrical test is passed, a separate shield cover is mechanically placed onto the frame.
Mechanical assembly of the cover onto the frame after re-flow soldering and testing may cause bending of the printed circuit board and excessive stresses within the board. These stresses may damage the printed circuit board conductors, connections, or components and lead to premature failure of the circuit board. Furthermore, signal leakage may occur during testing since the shield cover has not yet been inserted onto the frame.
The frame is commonly formed by stamping a metal plate that is typically thin. This often results in a frame which bends or bows after the cover is attached. The bending or bowing of the frame may result in the metal frame contacting internal components of the printed circuit board which can lead to electrical shorting of components of the circuit board. Manufacturers sometimes increase the cross-sectional thickness of the entire frame in an attempt to reduce bending of the frame. However, the thicker cross-section requires more material and provides minimal stiffness towards the center of the frame edges. Also, the increased cross-section does not add material to the corners of the frame which often exhibit twisting under load from the cover.
Another drawback with conventional frame designs is that the corners of the stamped parts do not typically provide sufficient shielding for high frequency signals. This is a result of the large gaps present in the frame corners due to design and stamping process limitations.
There is therefore a need for a shielding device that provides a frame with increased stiffness and corners designed to maximize contact between the frame and cover to reduce the occurrence of high frequency signals entering or escaping from the shielding device.